Tuesday, December 25, 2012
The Reno Accident
Meticulous probing of wreckage led investigators to some tiny screws
Investigators could not determine whether the airplane encountered wake turbulence from one of the other racers and it was responsible for beginning the upset. They did note that, regardless of whether the link assembly failure or a wake vortex encounter initiated the roll upset, the loss of the left trim tab's downward force on the elevator resulted in a sudden upward deflection of the elevator, a sudden and forceful aft movement of the control stick, the pitch-up of the airplane, and the rapid increase in G forces.
The screw looseness and cracking would have allowed for reduced stiffness of the trim tab system that could have allowed a flutter condition to develop. Flutter would be capable of producing sufficient dynamic loads to buckle and fracture the link assembly.
Investigators were unable to find documentation that all of the modifications had been subjected to complete engineering analysis and flight testing. The Safety Board raised the possibility that any adverse effects of modifications on the structure or flying characteristics could have been identified and corrected. The NTSB reported that neither the pilot's family members nor the airplane's ground crew were aware of any detailed drawings, engineering calculations, or other substantiating data for any of the modifications.
The NTSB determined that the probable cause of this accident was the reduced stiffness of the elevator trim tab system that allowed aerodynamic flutter to occur at racing speeds. The reduced stiffness was a result of deteriorated locknut inserts that allowed the trim tab attachment screws to become loose and to initiate fatigue cracking in one screw sometime before the accident flight. Aerodynamic flutter of the trim tabs resulted in a failure of the left trim tab link assembly, elevator movement, high flight loads and a loss of control. Contributing to the accident were the undocumented and untested major modifications to the airplane and the pilot's operation of the airplane in the unique air racing environment without adequate flight testing.
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